1. Field of the Invention
The present invention relates generally to the field of radiation imaging, and in particular to radiation imaging devices having an array of image cells.
2. Related Art
A variety of imaging devices comprising an array of image cells are currently known. A charged coupled image sensor (also known as a charged coupled device (CCD)) is one example of such an imaging device. A CCD type device operates in the following way. Charge is accumulated within a depletion region created by an applied voltage. For each pixel (image cell) the depletion region has a potential well shape and constrains electrons under an electrode gate to remain within the semiconductor substrate. Voltage is then applied as a pulse to the electrode gates of the CCD device to clock each charge package to an adjacent pixel cell. The charge remains inside the semiconductor substrate and is clocked through, pixel by pixel, to a common output. During this process, additional charge cannot be accumulated.
Another type of imaging device which is known is a semiconductor pixel detector which comprises a semiconductor substrate with electrodes which apply depletion voltage to each pixel position and define a charge collection volume. Typically, simple buffer circuits read out the electric signals when a photon is photo-absorbed or when ionizing radiation crosses the depletion zone of the substrate. Accordingly pixel detectors of this type typically operate in a pulse mode, the numbers of hits being accumulated externally to the imaging device. The buffer circuits can either be on the same substrate as the charge collection volumes, as disclosed in European Patent Application EP-A-0287197, or on a separate substrate that is mechanically bonded to a substrate having the charge collection volumes in accordance with, for example, the well known bump-bonding technique, as disclosed in European Patent Application EP-A-0571135.
Another type of imaging device is described in International patent application WO95/33332, which describes an Active-pixel Semiconductor Imaging Device (ASID). The ASID comprises an array of pixel cells including a semiconductor substrate having an array of pixel detectors and a further array of pixel circuits. The pixel detectors generate charge in response to incident radiation. Each pixel circuit is associated with a respective pixel detector and accumulates charge resulting from radiation incident on the pixel detector. The pixel circuits are individually addressable and comprise circuitry which enables charge to be accumulated from a plurality of successive radiation hits on the respective pixel detectors. The device operates, for example, by accumulating charge on a gate of a transistor. Accordingly, analog storage of the charge value is obtained. At a determined time, the charge from the pixel circuits can be read out and used to generate an image based on the analog charge values stored in each of the pixel circuits.
CCD devices suffer from several disadvantages, including limited dynamic range due to the limited capacity of the potential well inside the semiconductor substrate, and inactive times during which an image is read out. Pulse counting semiconductive pixel devices also suffer from limited dynamic range. As these devices read the pixel contact when a hit is detected, they suffer from saturation problems at high counting rates. The semiconductor pixel device according to WO95/33332 provides significant advantages over the earlier prior art by providing a large dynamic range for the accumulation of images.
However, CCD imaging devices and imaging devices of the type described in WO95/33332 suffer from a potential disadvantage in that the output signals from the individual pixel cells represent the accumulation of radiation intensity at that pixel cell between readout times. This means that radiation hits of varying energies could lead to an inaccurate count of the number of radiation hits. For example, a relatively small number of higher energy radiation hits would give the same output signal as a higher number of lower energy radiation hits (for example, scattered radiation hits).
Embodiments of the present invention seek to mitigate the problems of known imaging devices described above.